1. Field of the Invention
This invention relates to MR imaging apparatus using NMR (nuclear magnetic resonance), and more particularly to an MR imaging apparatus which acquires images by a technique called hybrid scan.
2. Description of the Related Art
According to a basic method, a conventional MR imaging apparatus executes a pulse sequence repeatedly. In the pulse sequence, one echo signal (primary echo) is generated by irradiating an examinee with one excitation RF (Radio Frequency) pulse (also called 90.degree. pulse since it rotates the spin phase of protons 90.degree.) and then one refocus RF pulse (also called 180.degree. pulse since it rotates the spin phase of protons 180.degree.). Subsequently, the pulse sequence is repeated with varied phase encoding amounts. Data acquired from the echo signal generated in one pulse sequence are arranged on one line in a raw data space (also called k space). In order to obtain a raw data space having 256 lines, for example, the pulse sequence must be repeated 256 times with different phase encoding amounts according to this basic method. Thus, data collection requires a time-consuming operation.
In view of the above situation, what is known as single-shot scan has been developed in which an examinee is irradiated with one excitation RF pulse (called a single shot), and data for all the lines in the raw data space are acquired with this single shot. The single-shot method includes a fast spin echo method and EPI (Echo Planar Imaging). The fast spin echo method uses a pulse sequence called RARE (Rapid Acquisition with Relaxation Enhancement) in which one excitation RF pulse is followed by a plurality of (e.g. 256) refocus RF pulses to generate echo signals which are subjected to different phase encoding. The echo planar imaging uses a technique called gradient switching in which one excitation pulse is followed by alternate reversals of the polarity of gradient field pulses effected plural times (e.g. 256 times) to generate a plurality of echo signals which are subjected to different phase encoding. However, the plurality of echo signals generated by these methods attenuate with time, and the later echo signal has the less strength. A sectional image of poor quality is reconstructed by arranging the data from these echo signals on a plurality of lines in a raw data space.
Then, a combination of the above basic method and single-shot scan method, known as hybrid scan method, has been developed. This hybrid scan method will be described, taking a combination of the basic method and fast spin echo method for example. In this method, four echo signals are successively generated by irradiating an examinee with one excitation RF pulse and then four refocus RF pulses. Each of these echo signals is subjected to four different types of phase encoding, to collect data for four of the 256 lines in the raw data space at a time. This pulse sequence is repeated 64 times to acquire data for all the lines in the raw data space.
Use of this hybrid scan method realizes a reduction in the number of times the pulse sequence is repeated in order to acquire necessary data for one raw data space without a substantial deterioration in image quality. Thus, an image pickup operation may be expedited.
However, the hybrid scan method has the following drawbacks.
Particularly in the hybrid scan method combining the basic method and fast spin echo method, i.e. where a plurality of refocus RF pulses are used to acquire a plurality of echo signals, and where CPFH (Carr-Purcell-Freeman-Hill) (also called modified CP) pulse sequence is used which alternately switches the phase polarity of the refocus RF pulses, the spin phase of protons is rotated 90.degree. by one excitation RF pulse, thereafter the spin phase of protons is rotated 180.degree. by the first refocus RF pulse, and further the spin phase of protons is rotated -180.degree. by the second refocus RF pulse. In this way, the phase polarity is switched a predetermined number of times, alternately for the refocus RF pulses odd-numbered and even-numbered in order. Each of the echo signals thereby generated is subjected to different, continuous phase encoding. The data derived from the echo signals are arranged along adjacent lines, i.e. lines with continuous phase encode amounts, in the raw data space. An image is reconstructed by effecting a Fourier transform on this raw data space. The phase encode amounts of the adjacent lines which should be continuous are discontinuous because of the phase difference (180.degree.) between the echo signals. A blur which is one type of artifacts occurs to the image reconstructed by a Fourier transform of the raw data space having such discontinuous phase encode amounts.
Where refocus RF pulses are used likewise and CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence is used in which the refocus RF pulses are emitted in a phase 90.degree. different from that of the excitation RF pulse, no phase difference occurs between the echo signals since the refocus RF pulses have no phase difference therebetween. Consequently, a blur does not occur to one raw data space, i.e. one slice image reconstructed. However, when data are collected for a plurality of raw data spaces (i.e. for multiple slices), a phase difference occurs between adjacent lines in the raw data spaces for those of the slices other than the central slice, i.e. peripheral slices, for the following reason.
For obtaining multiple slices, it is usual practice to apply slice-selecting gradient field pulses along a direction of thickness of the slices. The slice-selecting gradient fields pulse are applied to have zero strength for the central one of the slices. A carrier frequency of the excitation RF pulse and refocus RF pulses is deviated by an amount corresponding to a slice location from a frequency for selecting the central slice for which the gradient field pulse has zero strength. While a plurality of slices are selected in this way, the carrier frequency of the excitation RF pulse and refocus RF pulses is different between the central slice and peripheral slices. However, the echo signals collected from the plurality of slices are detected as data by a phase detection using a reference frequency which is the same as the carrier frequency for selecting and exciting the central slice. Since the carrier frequency used in emitting the RF pulses to the peripheral slices is different from the reference frequency for detecting the echo signals generated by the RF pulses, the phase encode amounts arranged along the lines in the raw data spaces for the peripheral slices shift by phase differences corresponding to the above frequency differences. That is, phase differences occur to the peripheral slices since a rotatory coordinate system of the carrier frequency rotates relative to a rotatory coordinate system of the reference frequency which is an observation system. These phase differences result in blurs occurring to reconstructed images as noted above.
A hybrid scan method combining the basic method and echo planar method has the following drawback.
In this case, echo signals are generated by polarity reversal of the gradient field pulses, and differences in echo peak generating time result from differences in timing of polarity reversal of the gradient field pulses and non-uniformity of the static magnetic field. That is, phase differences occur among the echo signals. This also causes a blurred image.
It is conceivable to avoid the phase differences among the echo signals noted above by varying timing of refocus RF pulse emission, thereby controlling the peak generating time for each echo signal. However, this would require a control system of an RF emitter and the like to effect high-speed controls of the frequencies and emission timing of the refocus RF pulses, thereby imposing a heavy load on the control system. This measure is impractical in that the high-speed controls would tend to involve insufficient precision and require stability of the control system.